Luteinizing hormone/chorionic gonadotropic (LH/CG) receptors on the corpus luteum and on Leydig cells bind luteinizing hormone (LH) and human chorionic gonadotropin (hCG) to regulate steroid synthesis and secretion from these glands. The goal of this project is to better understand the role of LH/CG receptor structure in regulation of fertility and how gonadotropins may modulate events in LH/CG receptor function including G-protein interactions, receptor internalization and CAMP activation. First, we will examine whether gonadotropin binding causes size changes in functional LH receptor complexes using two laser optical methods for measuring protein molecular motions, polarized fluorescence depletion and fluorescence photobleaching recovery. Single-cell fluorescence energy transfer measurements will be used to assess receptor-receptor aggregation on the membrane. Second, the effect of LH/CG structure on receptor molecular motions will be assessed using tissue-cultured cells bearing LH/CG receptors with C-terminal truncations. Ligand binding to these truncated receptors results in increased CAMP activation and more rapid internalization, perhaps due to more rapid LH/CG receptor lateral diffusion in the plasma membrane. Third, the effects of deglycosylated hCG and native hCG on LH/CG receptor motions will be evaluated on rat Leydig cells and on MA-10 cells. Binding of deglycosylated hCG to the LH/CG receptor slows receptor down-regulation and internalization, effects that may result from slower receptor lateral diffusion and fewer receptor interactions with proteins needed for internalization. Finally, the association of non-receptor proteins including G-protein subunits and MHC class I antigen with LH/CG receptors will be explored by a photoproximity labeling technique that permits identification and isolation of proteins physically proximal to a known membrane site. Colorado State University has an unusual confluence of expertise is reproductive physiology and instrumental facilities for membrane structural studies. This situation provides us a unique opportunity to study both LH receptor organization and how this organization modulates receptor function.